The present invention generally relates to a high resistivity CZ silicon structure, and a process for the preparation thereof. In particular, the high resistivity silicon structure comprises a large diameter CZ silicon wafer, or a substrate derived therefrom, wherein the resistivity of the wafer or substrate is decoupled from the concentration of acceptor atoms (e.g., boron) therein, the resistivity of the structure being substantially greater than the resistivity as calculated based on the concentration of said acceptor atoms therein. Such a wafer is particularly well-suited for use in, for example, high frequency (e.g., microwave or RF) applications.
Traditionally, gallium arsenide (GaAs) wafers have been mostly used in high resistivity devices. Gallium arsenide not only has the advantage of a naturally high carrier mobility, but it also offers the possibility of high resistivity substrates which are required for device isolation and minimization of substrate cross-talk, transmission line loss and making high-Q inductors in radio frequency applications and monolithic circuits.
Recently, however, as advances in the manufacturing technology for the preparation of high resistivity single crystal silicon wafers have been achieved, the use of such wafers in the high resistivity electronics industry has expanded. Two methods are used to manufacture single crystal silicon: the Czochralski (CZ) method and the floating zone (FZ) method. Although FZ silicon is commercially available having resistivities of up to about 10 kohm-cm or more, this material has limitations. For example, such material is expensive to manufacture, it lacks the mechanical stability needed for many applications, at least in part due to the low oxygen content therein, and it is of limited size. For example, it is not available in diameters of 300 mm or more, which is the developing industry standard.
Although CZ silicon addresses many of the limitations associated with FZ silicon, CZ silicon prepared by current techniques is also not without limitations. For example, boron is a common contaminant in CZ silicon. In order to grow CZ material of sufficiently high purity to achieve such high resistivities directly, boron concentrations may typically not exceed 1.3×1013 atoms/cm−3. Manufacturing CZ silicon in commercial environments to this level of purity, or beyond, is difficult and expensive. For example, typically fully synthetic crucibles are needed. However, once such a low boron concentration is achieved, a second challenge exists, that being the presence of thermal donors. Thermal donors are produced during the thermal treatments employed as part of the integrated circuit manufacturing process, as a result of the presence of interstitial oxygen in the CZ silicon.
The formation of thermal donors is generally not problematic in low resistivity wafers because the residence time in the greater than 300 and less than 500 C temperature range within which they typically form, is relatively short, typically about one to two hours, and the majority carriers, introduced in n-type or p-type doping, will normally dominate. For high resistivity applications, where the added dopant concentration is low, however, the formation of thermal donors in the device processing steps is a major factor in final wafer resistivity. (See, e.g., W. Kaiser et al, Phys. Rev., 105, 1751, (1957), W. Kaiser et al, Phys. Rev., 112, 1546, (1958), Londos et al., Appl. Phys. Lett., 62, 1525, 1993.) Thus, for high resistivity CZ applications, residual interstitial oxygen concentration will strongly influence the rate of thermal donor formation during device processing.
To-date, solutions proposed to the CZ thermal donor problem have essentially involved the same approach; that is, these approaches attempt to suppress the oxygen content in the silicon substrate far below that which is achievable by the CZ process alone. The idea here is that for every target value of initial substrate resistivity, there is an oxygen concentration sufficiently low, such that thermal donor generation will not be an issue. Typically, this approach involves thermal treatments to precipitate out of the solid solution the grown-in interstitial oxygen. However, this approach is costly and time consuming, involving long periods of time, typically tens of hours, at high temperatures.